To date, in distance detection systems, as methods of detecting the distance between terminals implementing wireless communication with each other, distance detection systems have been proposed that are adapted to utilize, for example, the first method (e.g., refer to Non-Patent Documents 1 to 3) in which, utilizing the fact that an electric wave (radiated electromagnetic field) is attenuated in linearly inverse proportion to a distance r, a receiving-side terminal detects the distance between a sending-side terminal and the receiving-side terminal, based on the electric-field intensity of a received signal or the second method (e.g., refer to Non-Patent Document 4) in which, with clock pulses of the terminals being precisely synchronized, the receiving terminal detects the distance, in accordance with a phase shift (delay component) of a pulse signal, or an M-sequence signal, sent from the sending-side terminal.    Non-Patent Document 1. GIDIN F (Silicon Graphics., CA), PANTIC-TANNER Z (San Francisco State Univ., CA): Analysis of the Measurement Uncertainty with 1/R Ex-tapolation of Radiated Emission Measurements on an Open Area Test Site (OATS)., IEEE Int Symp EIelctro-magn Compat, OL. 1998, NO. VoL. 1; PAGE. 137-140; 1998    Non-Patent Document 2. HASHIOTO H, YAMAZAKI M (Tokai Univ., Kanagawa, JPN): a distance sensor utilizing an M-sequence barcode as an indicator, J Adv Sci, Vol. 12, NO. 1/2; PAGE. 144-145; 2000    Non-Patent Document 3. Shibutani Akinori, Nakatsugawa Masashi, Umehira Masahiro (NTT Network Innovation Laboratories), Kubota Shuji (NTT Science and Core Technology Laboratory Group): Discussion About High-Precision Distance Detection by Normalized Least Squares Method, Research Report of Information Processing Society of Japan, VOL. 2001, No. 83 (MBL-18 ITS-6); PAGE. 9-14; 2001    Non-Patent Document 4. Koyama Shinji, Shimada Kanzo, Shiba Nobuyuki, Yasuda Akio (Tokyo University of Marine Science and Technology): GPS Positioning Computing Program, Technical Research Report of the Institute of Electronics, Information and Communication Engineers, VOL. 101, No. 33 (SANE2001 1-11); PAGE. 39-44; 2001
However, in a distance detection system that detects a distance by utilizing the first method, there has been a problem in that, because the electric-field intensity of a signal received by a receiving-side terminal is largely affected by various factors such as a distance loss due to electric-wave reflection on the ground, frequency, electric-wave polarization, the altitude of a radiating body, and shapes and directions of transmitting and receiving antennas, the relationship in which an electric wave is attenuated in linearly inverse proportion to a distance is not retained, whereby accuracy of distance detection is deteriorated.
In contrast, in a distance detection system that detects a distance by utilizing the second method, there has been a problem in that, because, in order to keep clock pulses of a sending-side and a reception-side terminal always precisely synchronized, extremely complicated control, e.g., utilizing the GPS (Global Positioning System), and an apparatus for making the clock pulses synchronize with each other are required, a complex configuration is necessary, as a whole of the system.